The invention relates to a piston usable in a pressure operated dispensing container.
Pressure operated dispensing containers which utilize a piston longitudinally slidable within the container are known in the prior art. These pressurized containers are used to dispense a variety of different materials of varying viscosities. The containers generally include a cylindrical can closed at one end and provided with a dispensing spout under the control of a valve. The opposite end of the container is sealed.
The piston is received within the container and serves to separate the container into two chambers. The product to be dispensed occupies the upper chamber, above the piston. A pressurized fluid which acts as a propellant, occupies the lower chamber, below the piston. The piston is roughly in the form of an inverted cup and has an upper and an annular skirt or sidewall which extends down from the upper surface. The upper surface acts as a barrier to separate the product and the propellant. The annular sidewall of the piston stabilizes and positions the piston in the container and provides a surface which rides on the inner wall of the container.
The product to be dispensed is loaded into the upper chamber of the container under pressure. The loading is a three stage operation. Each stage occurs at a different index position on the loading machine. During the first stage, known as the fill stage the product is introduced into the can above the top of the piston. During the second stage, known as the pressure stage a pressure differential is created above and below the piston to force some of the product down around the periphery of the piston between the piston sidewall and the container. During the third stage, known as the pushup stage, the piston is pushed toward the top of the container. This pushup stage also causes product to seep down around the periphery of the piston. After the loading of the product into the upper chamber is completed, propellant is loaded into the lower chamber under pressure. In use, when the valve at the top of the container is opened, the propellant pushes the piston toward the top of the container through the valve.
Prior art pistons have not been entirely satisfactory during both the loading of the pressurized container and during the dispensing of the product therefrom. During the pressure stage of the loading operation these pistons have tended to buckle-in, deform and tilt causing (a) loss of product down one side of the piston into the bottom of the container and (b) lack of seal on the other side of the piston resulting in excessive secondary permeation and/or bypass.
In loading a pressurized container it is cost-efficient to do so at high speed and using high pressure. Heretofore, the aforementioned problems with piston tilt and deformation have been avoided by loading containers at a less than efficient speed and/or pressure.
After the container is loaded the piston must be able to maintain the seal between its sidewall and the inner surface of the wall of the container. It must minimize secondary permeation which is diffusion of propellant around the piston at the propellant-product interface. This secondary permeation allows propellant and product to mix and thus decreases product shelf life and otherwise adversely affects the product. Further, during dispensing of the product, it is important that the piston minimize the bypass of propellant around the piston skirt into the product.
Piston skirt length is a function of container diameter. Although a piston which provides little clearance between itself and the container inner wall decreases secondary permeation, this type of fit increases bypass. As the piston diameter approaches that the container thereby decreasing clearance, the likelihood of secondary permeation around the piston obviously lessens. Further, for purpose of decreasing this secondary permeation the longer the length of a tight fitting piston the better. However, a piston which provides little clearance over a distance also increases resistance to movement. The increased resistance to movement results in increased bypass when the container valve is first opened. Accordingly, the most effective piston is one which has a diameter capable of minimizing secondary permeation without concomitantly creating a bypass problem within the confines of the piston length necessitated by the particular can.
An object of the present invention is to provide a piston which does not deform, tilt or shift when product is loaded into a container at high speed and under high pressure.
Yet another object of the present invention is to provide a piston which will minimize both secondary permeation and bypass.
A further object of the present invention is to provide such a piston as which will facilitate even distribution of product between the sidewalls of the piston and container.